1. Field of the Invention
The present invention relates to polyurethane resin powder enamels.
2. Description of the Background
In about 1970, an intense period of activity began for the development of a new "generation" of powders. These powders are the polyurethane (hereinafter "PUR") powders which are based on .epsilon.-caprolactam-blocked isophorone diisocyanate/polyol adducts and polyester polyols, as described, for instance, in, e.g., German OS No. 19 57 483 and Ger. Pat. No. 2,127,839. .epsilon.-Caprolactam-blocked isophorone diisocyanate (IPDI) melts at 53.degree.-55.degree. C. Because the blocked IPDI has such a low melting temperature, powders produced from this blocked IPDI agglomerate or cake during storage. In order to raise its melting point, IPDI has been subjected to chain lengthening with a polyol (NCO:OH=2:1) before being blocked with the .epsilon.-caprolactam-blocking agent. Polyols such as 3-hydroxymethyl-1,5-pentanediol, 2,2,4-trimethyl-1,6-hexanediol, and diethylene glycol are mentioned in German Pat. No. 2,105,777 as chain lengthening agents for IPDI, and German OS No. 25 42 191 refers to mixtures of di- and trifunctional polyols as chain lengthening agents for IPDI.
The enamel or varnish (hereinafter the term "enamel", will only be used but includes varnishes within its scope) coatings produced with heat by these .epsilon.-caprolactam-blocked IPDI/polyol adducts are distinguished by good flow properties, good chemical resistance, and other good properties (beside flow properties) relating to coating technology. A crucial disadvantage of such PUR powder enamels however, is their high curing temperature of about 200.degree. C.
There has been no lack of attempts to reduce the high curing temperatures of PUR powder. German OS No. 30 04 876 describes the preparation of blocked IPDI adducts which have greater reactivity than the PUR powder hardeners described in German Pat. No. 2,105,777, said preparation being by means of reaction with a polyol such that 8 equivalents of NCO in the IPDI reactant react per OH equivalent in the polyol, following which reaction the free NCO groups are blocked.
German OS Nos. 27 35 497 and 28 42 641 describe more reactive PUR powders which are characterized by having as a hardener an .epsilon.-caprolactam-blocked isocyanate-group containing IPDI adduct. The curing temperature of the PUR powders described in these patent-related publications is at least 170.degree. C.
A further reduction of the curing temperature of PUR powders has been found to be achievable by employing IPDI as a hardener and IPDI/polyol adducts (NCO:OH=2:1) having cyclic-amidine-blocked NCO groups (German OS No. 27 44 721). However, the cured powder enamel films have a pronounced yellow color.
There is currently a great demand for PUR powder hardeners from which weatherable, non-colored powder coatings can be produced at temperatures below 170.degree. C.
German OS No. 32 31 558 shows that it is known to employ secondary amines as blocking agents for isocyanates, in particular in the industrial area of solvent-containing varnishes, and the like. The use of amines for this purpose is attended by the problem that some amine always remains in the coating and leads to coloration (see German OS No. 32 31 558, p. 8 line 34, and German OS No. 30 39 824, p. 6 line 19).
Thus it is surprising that amines as blocking agents have proven suitable for use in the solvent-containing varnish sector. This suitability may be explained by the fact that the amines are nearly completely removed with the solvent.
A need therefore continues to exist for a blocked isophorone diisocyanate of improved properties.